Copper Metal Surface Research Articles

Remote Laser-Induced Breakdown Spectroscopy for the Detection and Removal of Salt on Metal and Polymeric Surfaces

The detection of contamination such as salt in outdoor high-voltage insulator systems and its subsequent removal are vital for a reliable transmission of electric power. Remote detection of salt on a copper metal surface was carried out by using a mobile laser-induced breakdown spectroscopy (LIBS) Lidar system with a laser wavelength of 355 nm. Detection of salt on a polymeric high-voltage insulator was obtained when an additional lens was inserted into the beam path, and the number of photons that was detected could be calculated by using a calibrated white light source. Ablative cleaning could readily be carried out with LIBS and was verified by observing the disappearance of the sodium D-line emission.

Electron gas-density functional calculations of the repulsive potentials between noble gas atoms and noble metal surfaces

An electron gas-density functional approach previously used to calculate the potential between two rare gas atoms has been applied to the calculation of the interaction potential of a rare gas atom with the surface of a noble metal. The method is illustrated by calculating the repulsive potential for helium atoms interacting with copper and silver metal surfaces assuming that the surface is either made up of discrete atoms or is a jellium continuum. The results compare well with published ab initio results. The theory provides estimates of the locations of the jellium edge and of the Nørskov parameter.

On-board production of hydrogen for fuel cells over Cu/ZnO/Al 2O 3 catalyst coating in a micro-channel reactor

Cu/ZnO/Al 2O 3 catalyst coatings have been prepared for a micro-channel reactor, which is more suitable for on-board production of hydrogen concerning steam reforming of methanol (SRM) for fuel cells compared to conventional systems. Several parameters that control the structural characteristics of the Cu/ZnO/Al 2O 3 catalyst coatings have been optimized in an annular micro-reactor that is similar to the micro-channel reactor in flow pattern, but is much simpler to be fabricated and can be used repeatedly. It was confirmed that catalytic activity was directly related to copper metal surface area. The catalyst COAT-14-6 (CuO 14 wt.% and ZnO 6 wt.%) that showed the highest copper area gave the best result for SRM. A micro-channel reactor was developed by diffusion bonding and wet chemical etching. The performance of the micro-channel reactor with the optimum Cu/ZnO/Al 2O 3 catalyst coating has also been investigated in a continuous reaction for 100 h. The result showed the micro-channel reactor could generate enough hydrogen for power output of 10 W.

Passivation phenomena during sonovoltammetric studies on copper in strongly alkaline solutions

In alkaline solution a strongly passivating layer is formed close to a copper metal surface during a positive sweep to potentials beyond approximately +0.5 V (vs. SCE). Simultaneous ultrasonic irradiation during the voltammetric scan promotes the anodic dissolution of copper and greatly increases anodic currents in the less-positive potential region 0–0.5 V (vs. SHE), which corresponds to the formation of a thicker, more porous oxide/hydroxide layer, but does not prevent this passivation process from occurring at potentials above 0.5 V in sodium hydroxide solution up to 3 mol dm −3 concentration. The passivating layer requires maintenance of oxidising conditions, and so the removal of the protective oxide/hydroxide coating, which is effectively performed by ultrasound, facilitates reductive loss of the passivating layer sufficiently readily that an anodic current can be seen during the reduction scan between +0.5 and 0 V while scanning from the positive limit in a negative direction. In 1 M NaOH a curve-crossing phenomenon occurs in silent voltammetry in which scan reversal before the onset of passivation produces a higher anodic current in the reverse (reductive) direction than during the forward positive scan before reversal. Also at this alkali concentration, when the positive range is scanned into the passivation region the maximum anodic peak current in silent voltammetry is independent of scan rate over the range from 5 to 80 mV s −1. Presonication of the copper surface prior to silent voltammetry greatly increases the peak currents and enhances peak definition in the copper voltammogram, suggesting general activation of the metal surface by ultrasound. The oxidation processes continue to occur at similar potentials, but there is a new peak in the reduction scan, with some shift of reduction potentials.

The Compatibility of Copper CMP Slurries with CMP Requirements

Cu is currently used as a replacement for aluminum in IC interconnections. One of the challenges of integrated circuit (IC) interconnection technology is the planarization process that involves fine copper lines patterning; the most perspective technique for Cu lines patterning is chemical mechanical planarization (CMP). CMP is applicable only once a given metal-slurry system can provide certain requirements. Different media, such as ammonium hydroxide peroxide ferric nitrite and nitric acid with and without the presence of inhibitors, were suggested as potential candidates for copper CMP slurries. In this work we studied the compatibility of these media with CMP requirements. Our work indicated that these slurry solutions (basic and acidic media) do not provide sufficient conditions for a conventional CMP application. The Cu is actively dissolved in all of these solutions and in some of them with high dissolution rate, without a rapid repassivation. The sharp acceleration in Cu etching rate under abrasive abrading can be attributed only to a mechanical Cu removal. It was found that the active dissolution of Cu is conducted through deep intergranular penetrations which may result in the deterioration of thin Cu connectors. This study also shows that the use of benzotriazole or other inhibitors commonly used for reducing the etching rate of the actively dissolved Cu cannot provide the needed conditions for a rapid passive film growth on the copper metal. The electrochemical behavior of Cu in these solutions, morphology of dissolved copper metal surface, and mechanism of copper CMP are discussed. © 2003 The Electrochemical Society. All rights reserved.

Mechanistic aspects of hydrogen production by partial oxidation of methanol over Cu/ZnO catalysts

In this work, mechanistic aspects of the partial oxidation of methanol (POM) to hydrogen and carbon dioxide over Cu/ZnO catalysts have been investigated. The data obtained with different catalyst compositions and different Cuo metal surface areas showed that the reaction depends on the presence of both the phases ZnO and Cuo. On the other hand, for catalysts with Cu concentrations in the range 40-60 wt%, the copper metal surface area seems to be the main factor determining the reaction rate. Kinetic isotope effects using CH3OH and CH3OD showed that both C–H and O–H bonds are at least partially involved in the rate-limiting step. TPD experiments with pure Cuo, pure ZnO and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface methanol can form intermediates which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H2, HD, D2, H2O, HDO and D2O in the temperature-programmed reaction of CH3OD revealed a slight enrichment of the products with H, suggesting that during methanol activation on the ZnO some of the D atoms might be retained by the support. The effect of oxygen partial pressure suggests that oxygen atoms on the copper surface strongly promote methanol activation and H2 and CO2 formation. It is proposed that oxygen atoms participate in methanol activation by the abstraction of the hydroxyl H atom to form methoxide and OHsurf. This OHsurf species rapidly loses H to the surface regenerating the Osurf.

Benzotriazole maleimide as a bifunctional reactant for SERS

The synthesis of a benzotriazole maleimide is reported and its use as a bifunctional compound demonstrated. The triazole moiety of the compound complexes strongly with metals such as copper and silver and can be used to form monolayers on metal surfaces. The maleimide acts as a dienophile and reacts with dienes to produce cycloadducts. We report the selective reaction of the benzotriazole maleimide with seven different dienes to produce a range of cycloadducts. These cycloadducts were then adsorbed onto a metal surface via the triazole group. The presence on the metal surface was confirmed by surface enhanced Raman scattering, SERS. SERS is a vibrational spectroscopy and as such provides a fingerprint of each compound examined. The cycloadducts all gave different spectra that allowed identification of each diene that had cyclised. The dienes did not produce SERS on their own and had to be reacted with the bifunctional benzotriazole maleimide prior to examination. This provides an illustration of the use of bifunctional reactants specifically designed to produce SERS active products and also provides an example of an efficient derivatisation chemistry for copper and silver metal surfaces.

X-ray photoelectron spectroscopy and Auger electron spectroscopy investigation on the oxidation resistance of plasma-treated copper leadframes

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been employed in the investigation of the surface chemistry and oxidation-resistant property of the C-, N-, and Cu-containing films, formed by the plasma treatment of pristine copper. The copper oxidation process has been elucidated using realistic microelectronic packaging-assembly conditions, results of which are in agreement with established models. From the chemical shift in binding energy and change in full-width-half-maxima of the XPS core-level peaks, the heated copper has been observed to first form the monovalent cuprous oxide (Cu2O) followed by nucleation and growth of the divalent cupric oxide (CuO) above the Cu2O. The copper hydroxide [Cu(OH)2] has been observed to form on the Cu2O, CuO, and copper metal surfaces. This knowledge of copper oxidation has been used as a yardstick for the study of film decomposition and subsequent oxidation of exposed copper. Variation of the plasma treatment time produced two films of varied thickness [CuCxNy(A):15 nm and CuCxNy(B):75 nm], with the longer time producing the thicker films. AES results also suggest that the two films have the analogous film stoichiometry of 1:1:0.16 (Cu:C:N). Further XPS and AES analyses reveal the thickness dependence of the oxidation-resistant property, with the thicker film having the better oxidation resistance. A difference in film thickness of ∼5 times delayed the film decomposition temperature by 50 °C. AES depth profiles on the heated (250, 300, and 350 °C) films show the gradual decay of the film thickness with heating temperatures. Oxide growth predominates with the complete decomposition of the films. Present results suggest neither the presence of entrapped N2 gas in the copper oxide structure nor an oxide structure with the C and N decorations at the copper oxide grain boundaries. One possible film decomposition mechanism may be through the breaking of the Cu–N bonds under suitable heating conditions, thereby allowing the formation of the Cu–O bonds as the oxidation proceeds.

The flexible surface or the rigid surface

In this paper we discuss aspects of the concept described by Somorjai as the “flexible surface”, and whether some surfaces can be considered to be inflexible, or rigid. We present STM results which appear to manifest both types of behaviour for surfaces, depending on their oxidation state. Copper metal surfaces can be classed as flexible, showing facile reconstruction in the presence of oxygen, whereas an oxidised Pd(110) surface shows no apparent diffusivity, even at 500 K. We go on to show data for a bulk oxide which indicates that sub-stoichiometry in the sample induces an element of flexibility in the surface, especially during reaction with oxygen. Finally, this is related to the direct observation of spillover on model catalysts of Pd nanoparticles supported on TiO2. It must be recognised that flexibility relates to surface diffusivity and hence length- and time-scales. Surfaces which appear inflexible at short times may be flexible at long times. In relation to catalysis then, surface flexibility depends on the relationship between the time-scale of diffusive events on the surface and the catalytic turnover number.

Modification of Cu/ZnO/SiO 2 catalysts by high temperature reduction

In this overview results obtained in three laboratories on Cu/ZnO/SiO 2 catalysts prepared by homogeneous deposition–precipitation are summarised. It is concluded that the system adopts a layered ‘zincsilite’ structure. In this copper–zinc hydrosilicate the Cu and/or Zn cations are located in octahedral sites and in the interlayer. When copper or zinc is present in excess of the zincsilite ratio (Cu y + Zn 1− y : Si = 0.75) or at relatively high copper content ( y ≥ 0.40), copper–zinc and copper hydroxocompounds are formed, the latter yielding CuO after calcination. Cu/ZnO/SiO 2 shows increased activity in ester hydrogenolysis and methanol synthesis reactions as a function of reduction temperature, in contrast to the unpromoted catalyst. Inert treatment at 700 K significantly decreases activity, whereas subsequent reductive treatment restores the high activity mode. The copper metal surface area of the promoted catalyst increases modestly with reduction temperature when the catalyst is carefully evacuated preceding a N 2O chemisorption measurement. Significantly lower surface areas result after prolonged evacuation at higher temperatures. It was shown using infrared spectroscopy that upon high temperature (700 K) reduction cationic copper sites are created. Investigation of 63Cu/ 68ZnO/SiO 2 by low energy ion scattering demonstrates that the surface of the catalyst reduced at 700 K is enriched in zinc. In the 600 K reduced catalyst the zinc enrichment was less prominent. It is proposed that the Cu–ZnO interface plays a crucial role in catalysing the reactions studied. It is not yet possible to decide between two models arriving at a situation where this interface is enlarged: (1) Reversible formation of flat epitaxial particles upon high temperature reduction from the fraction of the copper dissolved in the mixed oxide catalyst precursor phase. (2) Migration of partly reduced ZnO on top of copper.

Ester hydrogenolysis over promoted Cu/SiO 2 catalysts

A series of Cu/SiO 2 catalysts promoted with manganese, iron, cobalt, nickel, molybdenum, magnesium and yttrium was prepared by homogeneous deposition precipitation. For most catalysts a copper metal surface of about 15 m 2 g −1 catalyst is measured. TPR reveals that copper–promoter contact is good for most samples. Despite similar Cu 0 surface areas, ester hydrogenolysis activity varies considerably with the type of promoter, in the following order: Mo>Co≥Zn≥Mn>Fe≥Y>Ni≫Mg. As a function of metal–oxygen bond strength, a vulcano type catalyst activity plot is observed. With respect to alkanes formation, a correlation with normalised electronegativity is observed, inferring that the formation of alkanes proceeds on Lewis acid sites, which is in agreement with literature results. When both activity and selectivity are taken into account, manganese and zinc are the promoters of choice.

The role of surface oxygen on copper metal in catalysts for the synthesis of methanol

Discrepancies in experimental measurements of adsorbed oxygen coverage on copper metal surfaces in working Cu/ZnO/Al2O3 catalysts are interpreted in terms of two types of adsorbed oxygen. The first, O(a), is identical with that observed in studies of single‐crystal copper surfaces. The second, O*(a), not seen in single‐crystal studies, is more strongly bonded to the metal surface. It is suggested that the adsorption sites of O*(a) contain Zn as well as Cu, from surface α‐brass (copper/zinc alloy) formation during catalyst reduction. Earlier experimental results on O(a) coverages on various supported copper catalysts are re‐assessed. Only catalysts containing Zn (or Ga) gave abnormally high coverages: with other supports, basic or acidic, O(a) coverages are less than ∼0.1.

Reactions of Hydrogen Atoms with Solid, Thin Films of Acetone and 2‐Propanol at 13 K

In the interstellar clouds, a number of oxygen-containing organic compounds such as alcohols and carbonium compounds have been observed. In order to investigate the role of grain processes for the formation and destruction of these compounds, thin films of acetone and 2-propanol deposited on the copper metal surface at 13 K were reacted with cold gas-phase H atoms (~27 K). The major products from acetone were found to be methane and alcohols (smaller than propanol). 2-propanol was not detected as a reaction product. This indicates that the disproportionation reaction is the major process, but that the recombination one is minor in the reaction of H with the intermediate (CH3)2COH radical. The reaction of H with 2-propanol led to the formation of methane, alcohols, and acetone as major products. The dependences of the film thickness, reaction time, and reaction temperature on the product yields were measured. It is suggested that the diffusion length of the H atoms in the 2-propanol film is longer than that in the acetone film, and consequently greater product yields were obtained for 2-propanol than for acetone with thicker sample films. The temperature-independent product yields in the range of 13-80 K invoke the tunneling processes in the reactions.

Partial oxidation of methanol to produce hydrogen over CuZn-based catalysts

In this work Cu ZnO and Cu/ZnO/Al 2O 3 catalysts have been studied for the partial oxidation of methanol with oxygen to produce hydrogen. These CuZn based catalysts showed high activity for the partial oxidation of methanol and it was found that the catalytic activity is directly related to the copper metal surface area. In the series CuZn with copper relative content of 20–70 wt%, the catalyst Cu 40Zn 60 (Cu 40 wt% and Zn 60 wt%) which showed the highest copper area gave the best results for the partial oxidation of methanol. The activation energies and TOF (turnover frequencies) varied with the CuZn catalyst composition. For catalysts with low copper loading very high E a and TOF were obtained (for Cu 30Zn 70 E a=482 kJ/mol and TOF ca. 200 min −1 at 497–499 K) whereas for higher copper contents the E a and TOF decreased tending to constant values (for Cu 70Zn 30 E a=71 kJ/mol and TOF=160 min −1 at 497–499 K). These results are discussed in terms of a possible effect of the CuZnO interaction which depends on the catalyst composition. Catalytic experiments with Cu 40Zn 55Al 5 (Cu 40 wt%, Zn 55 wt% and Al 5 wt%) showed that the presence of aluminium has an inhibiting effect producing slightly lower methanol conversion. On the other hand, higher selectivities for H 2 and CO 2 were obtained with only traces of the undesirable carbon monoxide. Moreover, the Al is very important for catalyst stability and life-time experiments showed that Cu 40Zn 55Al 5 is stable during the partial oxidation of methanol with no significant change in activity and selectivity even after 110 h operation at 503 K. The catalyst Cu 40Zn 60 with no Al, deactivates rapidly after 20 h reaction at 503 K. Experiments using N 2O as oxidant showed higher activity to convert methanol but producing large amounts of H 2O and CO. The impregnation of catalyst with Na produced similar effect increasing the selectivity for H 2O and CO. The results presented seem to indicate that the copper metal is active for partial oxidation of methanol to H 2 and CO 2 whereas Cu +1 favour the formation of H 2O and CO. Cu +2 as CuO shows very low activity for methanol conversion producing only CO 2 and H 2O.

Activity and structure-sensitivity of the water-gas shift reaction over Cu [sbnd]Zn [sbnd]Al mixed oxide catalysts

The activity and structure-sensitivity of the water-gas shift (WGS) reaction over Cu Zn Al mixed oxide catalysts were studied. Three sets of samples with different Cu/Zn and (Cu+Zn)/Al atomic ratios were prepared by coprecipitation. Depending on the cation ratio, the ternary hydroxycarbonate precursors contained hydrotalcite, aurichalcite and/or rosasite phases. The decomposed precursors contained CuO, ZnO, ZnAl 2O 4, and Al 2O 3. The relative proportion of these phases depended on both the chemical composition of the sample and the calcination temperature employed for decomposing the precursor. After activation with hydrogen, samples were tested for the WGS reaction at 503 K. The turnover frequency of the eighteen samples tested was essentially the same (0.2–0.3 s −1) irrespective of changing the copper metal surface area between 3 and 35 m 2/g Cu and the metallic copper dispersion between 0.5 and 5.0%. This indicated that the WGS reaction is a structure-insensitive reaction, as the specific reaction rate r 0 (mol CO/h/g Cu) is always proportional to the copper metal surface area. Preparation of mixed oxides with a high copper dispersion is therefore required for obtaining more active catalysts. It was found that the value of the metallic copper dispersion is related to the amount of hydrotalcite contained in the hydroxycarbonate precursor: the higher the hydrotalcite content in the precursor, the higher the copper metal dispersion in the resulting catalyst and, as a consequence, the higher the catalyst activity. Ternary Cu/ZnO/Al 2O 3 catalysts exhibited a substantially faster WGS activity than binary Cu/ZnO catalysts. The addition of aluminium, although inactive for the WGS reaction, is required for improving the catalyst performance.

The reactions of carbon dioxide with the copper metal surface in a Cu/ZnO/Al 2O 3 catalyst

Recent XPS studies of the reactions of CO 2 on a polycrystalline copper metal surface cast new light on earlier results from the reactions of CO 2 with the copper metal surface in a commercial Cu/ZnO/Al 2O 3 catalyst. The catalytic results are now re-interpreted in terms of the formation of two types of surface carbonate, both of which decompose above about 350 K in the absence of CO 2 to leave O(a) on the Cu surface. The enthalpy change in the interconversion of the carbonates is about 18 kJ mol −1. From the limits on adsorption it is estimated that one form of adsorbed carbonate occupies 1 (or possibly 2) Cu metal surface atoms whereas the other form occupies more than 5 Cu metal surface atoms. These results are used to show that a recent interpretation of the observed high coverages of O(a) after methanol synthesis on Cu/ZnO/Al 2O 3 catalysts is not valid.

Relationship between the copper surface area and the activity of CuO/ZnO/Al 2O 3 water-gas shift catalysts

The catalytic activities and copper metal surface areas of CuO/ZnO/Al 2O 3 water-gas shift catalysts have been determined. There is no single function which can describe the relationship between the copper metal area and the activity of every catalyst. However, discrete copper area-activity relationships are evident within groups of catalysts whose precursor species were formed under identical preparation conditions. Within these groups simple, linear, area-activity relationships are discovered.

Structure-activity relation and ethane formation in the hydrogenolysis of methyl acetate on silicasupported copper catalysts

The activity and selectivity of silica-supported copper catalysts in the hydrogenolysis of methyl acetate were studied in relation to the structure of the active phase. The activity was found independent of the copper loading, when the weight of copper in the reactor was kept constant. Copper catalysts were prepared by homogeneous precipitation. In the range of reduction temperatures between 543 K and 743 K, the activity was constant, in spite of considerably different copper metal surface areas. The absence of a correlation between activity and copper surface area is assigned to the formation of highly dispersed copper not active in methyl acetate hydrogenolysis. In contrast to the equal activity, catalysts of low copper loading produced more ethane than the higher loaded catalysts, based on the weight of copper. A remarkable parallel between the amount of ammonia chemisorbed on the reduced catalyst and the ethane selectivity suggests that a dehydration reaction to ethene on acid sites located at the silica support, followed by hydrogenation on the copper metal phase, is responsible for the formation of ethane.

A motion-induced sub-surface deformation wear mechanism

A sub-surface deformation phenomenon was identified as a wear mechanism in the plated metallurgies of electrical contact tabs on printed circuit cards. Under vibratory sliding conditions, the plastic flow of the copper substrate produces a bending and thinning of the intermediate layer of nickel and causes an upsurgence or upward motion of the copper, nickel and noble metal surface layers. This results in a high spot on the surface which now experiences increased local wear. Ultimately, there is local exposure within the wear region of the nickel and copper to the ambient environment, providing a potential for corrosion. The resultant appearance of the surface and cross-sections through the wear scar indicate that the sub-surface layers are locally being driven through the surface layer. An additional related phenomenon was observed in the analysis of card tabs that were tested at an elevated temperature. The copper substrate, directly under the wear track, experienced recrystallization and grain growth; the normally mottled grain structure was replaced with large, well defined grains. The physical evidence for these mechanisms, along with quantitative results showing differences in wear rates when these phenomena occur and when they do not, are presented. In addition, associated friction behavior is presented.

Axial and azimuthal angle determination with surface-enhanced Raman spectroscopy: thiophenol on copper, silver, and gold metal surfaces

Axial and azimuthal angle determination with surface-enhanced Raman spectroscopy: thiophenol on copper, silver, and gold metal surfaces